The Digital Subscriber Line (xDSL) technique is a high speed data transmission technique that transmits data through twisted pair telephone lines (i.e., Unshielded Twist Pair (UTP) lines), where “x” represents a collection of digital subscriber loop techniques. The XDSL technique includes not only baseband DSL techniques, such as ISDN Digital Subscriber Line (IDSL) and Single-pair High-bit-rate Digital Subscriber Line (SHDSL), but also pass-band XDSL techniques, such as Asymmetric Digital Subscriber Line (ADSL) that utilizes the Frequency-Division Multiplexing (FDM) technique to enable coexistence of xDSL and legacy telephone service (POTS) in the same twisted pair line. xDSL occupies a high frequency band, while POTS occupies a baseband below 4 KHz, and the POTS signals are separated from the xDSL signals by means of a separator. The pass-band XDSL employs Discrete Multi-Tone (DMT) modulation, and a system that provides multi-channel xDSL access is referred to as a DSL Access Multiplexer (DSLAM).
Through years of development, the ADSL technique has evolved from the first generation to the second generation ADSL2, ADSL2+, and the newer second generation VDSL2. The number of frequency bands used by pass-band xDSL is increasing gradually, and accordingly, the bandwidths are widened gradually. For example, ADSL and ADSL2 downlinks employ a frequency spectrum below 1.1 MHz, and can provide up to an 8 Mbps downlink data rate. ADSL2+ widens the downlink bandwidth to 2.2 MHz, and therefore can provide up to an 24 Mbps downlink data rate. VDSL2 even uses a frequency spectrum as high as 30 MHz, and accordingly can provide up to a 100 Mbps symmetric uplink/downlink data rate.
As the frequency band used by xDSL technique increases, the crosstalk among xDSL service ports on the same UTP line, especially the crosstalk in the high frequency band, becomes more and more severe. Because FDM is used for uplink and downlink XDSL channels, Near-End Crosstalk (NEXT) usually does not severely degrade system performance. However, Far-End Crosstalk (FEXT) will severely degrade UTP transmission performance.